1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface rock formations. More specifically, the invention relates to techniques for safely drilling wellbores through limited volume hydrocarbon-bearing rock formations using dynamic annular pressure control systems.
2. Background Art
A drilling system and methods usable with the present invention are described in U.S. Pat. No. 7,395,878 issued to Reitsma et al. and incorporated herein by reference. During drilling, particularly in certain offshore formations, small-extent hydrocarbon bearing formations (“nuisance hydrocarbon formations”) are encountered. Initially, these hydrocarbon bearing formations may have hydrocarbon pressure in the pore spaces that exceeds the hydrostatic pressure of fluid in the wellbore. However, as hydrocarbon enters the wellbore, such formations lose pressure relatively quickly, because their areal extent is limited. Drilling through such nuisance hydrocarbon requires an optimum method to deplete the hydrocarbon volume and pressure to acceptable levels to continue drilling safely because such nuisance hydrocarbon zones are typically quickly depleted as a result of the release of hydrocarbons into the wellbore. Thus, it is not advisable to increase the density of the drilling fluid, or to use the so-called “Driller's method” of wellbore pressure control, which requires the standpipe pressure (i.e., the drilling fluid pressure as it is pumped into the drill string) to remain constant. The foregoing statements are also applicable to drilling hydrocarbon wells “underbalanced”, wherein the wellbore hydrostatic (and hydrodynamic) fluid pressure is maintained below the hydrocarbon fluid pressure in the pore spaces of the hydrocarbon bearing rock formations.
There is a need for a more efficient technique to drill through nuisance hydrocarbon and/or underbalanced drilling.